The invention relates to a pressure sensor having a housing, a process connection which is connected to the housing and serves to feed a medium whose pressure is to be measured to a membrane which is arranged in the housing, said membrane, in operation, undergoing a deflection which depends on the pressure which is to be measured, and a seal which seals a gap found between the housing and the process connection and/or the membrane.
Pressure measurement uses, for example , arrangements measuring the absolute pressure, relative pressure and differential pressure. In the case of arrangements measuring the absolute pressure, the pressure which is to be measured is sensed in absolute form, i.e. as a difference in pressure with respect to a vacuum. With an arrangement measuring the relative pressure, a pressure which is to be measured is recorded in the form of a difference in pressure with respect to a reference pressure, for example a pressure prevailing at the location of the sensor. In most applications, this is the atmospheric pressure at the location of use. Therefore, in the case of arrangements measuring the absolute pressure, a pressure which is to be measured is sensed with reference to a fixed reference pressure, the vacuum pressure, and in the case of arrangements measuring the relative pressure, a pressure which is to be measured is sensed with reference to a variable reference pressure, for example the ambient pressure. An arrangement measuring the differential pressure senses a difference between a first pressure and a second pressure fed to a differential-pressure measuring cell.
These arrangements for measuring pressure can all be realized using pressure sensors of the type mentioned at the beginning.
In order to be able to cover the greatest possible spectrum of possible uses, for example in chemistry and in the processing industry, seals having a high chemical resistance are required. Examples of suitable seal materials are polyfluorocarbons, for example polytetrafluoroethylene (PTFE). However, most chemically highly resistant sealing materials are free-flowing and change their shape when exposed to great changes in pressure and/or temperature. In pressure measurement, there is the risk that seals made of these materials become deformed and leaky over time. This may be very hazardous depending on the use because, for example, very hot media which are under high pressure and are chemically very aggressive may escape through the gap between the housing and the process connection and/or the membrane.
U.S. Pat. No. 4,798,089 describes a pressure sensor having
a housing,
a process connection which is connected to the housing
and serves to feed a medium whose pressure is to be measured to a membrane which is arranged in the housing,
the membrane, in operation, undergoing a deflection which depends on the pressure which is to be measured, and
a seal
which fills a gap found between the housing and the process connection,
bears directly against the gap and covers the gap, and
is clamped between the housing and an outer, pressure-insensitive edge of the membrane.
The membrane is a metallic separating membrane of a pressure-transmitting means which is filled with a liquid which transmits a pressure acting on the separating membrane to a pressure-measuring cell. In order for it to be possible to use a seal which is made of a material which is free-flowing because of pressure and/or temperature, a metallic surround is provided which has a first annular surface bearing against the seal, is connected fixedly to the membrane by a weld and has a second annular surface which bears against the seal and exerts a spring force on the seal, which spring force acts in the direction of the gap.
Pressure measurement advantageously makes use of ceramic pressure-measuring cells, since ceramic pressure-measuring cells have a measuring accuracy which is stable over a very long time. One reason for this is the fixed ionic bonding of ceramic resulting in the material being very durable and hardly aging at all in comparison with other materials, for example metals.
Ceramic pressure-measuring cells have hitherto conventionally been clamped at an outer edge into a housing with a seal being connected in between. A pressure which is to be measured is fed to the membrane through an opening in the housing or a process connection connected to the housing, which is generally metallic. A pressure which is to be measured can be fed directly to measuring cells of this type. The use, for example of a pressure-transmitting means connected upstream is not conventionally provided in the case of these pressure-measuring cells.
Ceramic has a very high chemical resistance and it would therefore be of great advantage to be able to use the abovementioned, chemically highly resistant sealing materials also in the case of pressure sensors having ceramic pressure-measuring cells. However, a surround as is known from the abovementioned prior art cannot be used in conjunction with ceramic pressure-measuring cells, since the surround cannot be welded onto a membrane made of ceramic.
An object of the invention is to provide a pressure sensor having a housing, a process connection which is connected to the housing, and a seal which seals a gap found between the housing and the process connection and/or the membrane, which pressure sensor can have a seal made of chemically highly resistant material and in which the membrane can consist either of ceramic or of metal.
To this end, the invention involves a pressure sensor having
a housing,
a process connection which is connected to the housing
and serves to feed a medium whose pressure is to be measured to a membrane which is arranged in the housing,
the membrane, in operation, undergoing a deflection which depends on the pressure which is to be measured,
a seal
which seals a gap found between the housing and the process connection and/or the membrane,
which bears directly against the gap and covers the gap, and
which is clamped between the housing and the membrane, and
a spring which is two-legged in cross section
whose first and second legs enclose an acute angle,
whose first leg has a leg surface which faces away from the second leg, rests on a supporting surface and is connected fixedly to the latter, and
whose second leg has a leg surface which faces away from the first leg, bears in a planar manner against the seal and exerts a spring force on the seat which spring force acts in the direction of the gap.
According to one refinement, the seal consists of a fluorocarbon, in partIcular of polytetrafluoroethylene (PTFE).
According to a further refinement, the supporting surface is metallic, and the spring is connected to the supporting surface by a weld.
According to a further refinement, the spring consists of an elastic metal.
According to a first refinement, the membrane is a measuring membrane of a ceramic pressure-measuring cell, which measuring membrane is arranged in the housing. The process connection has, upstream of the measuring membrane, an annular recess which is bounded by a cylindrical circumferential surface and a radially inwardly extending shoulder surface adjacent thereto on its side facing away from the membrane, and the shoulder surface forms the supporting surface. The seal has a triangular cross section, and a first circumferential surface of the seal rests on an outer, pressure-insensitive edge of the measuring membrane, a second circumferential surface of the seal rests on the cylindrical circumferential surface, and a third circumferential surface of the seal bears in a planar manner against she second leg surface of the spring.
According to a second refinement, the membrane is a metallic separating membrane of a pressure-transmitting means arranged in the housing. The process connection has, upstream of the separating membrane, an annular recess which is bounded by a cylindrical circumferential surface and a radially inwardly extending shoulder surface adjacent thereto on its side facing away from the membrane, and the shoulder surface forms the supporting surface. The seal has a triangular cross section, and a first circumferential surface of the seal rests on an outer, pressure-insensitive edge of the measuring membrane, a second circumferential surface of the seal rests on the cylindrical circumferential surface, and a third circumferential surface of the seal bears in a planar manner against the second leg surface of the spring.
According to a third refinement, the membrane is a metallic separating membrane of a pressure-transmitting means arranged in the housing. The housing has, upstream of the separating membrane, a recess which is surrounded by a cylindrical circumferential surface. The process connection is arranged upstream of the cylindrical circumferential surface and has a radially inwardly extending shoulder surface which is adjacent to a side of the cylindrical circumferential surface which faces away from the membrane, and an outer, pressure-insensitive annular surface of the separating membrane forms the supporting surface. The seal has a triangular cross section, and a first circumferential surface of the seal rests on the shoulder surface, a second circumferential surface of the seal rests against the cylindrical circumferential surface, and a third circumferential surface of the seal bears in a planar manner against the second leg surface of the spring.
One advantage of the pressure sensor according to the invention is that a special form of the spring enables even materials which are free-flowing under certain circumstances because of pressure and/or temperature to be used for the seal. This is not only possible in conjunction with metallic membranes, but equally also in the case of ceramic membranes without other forms of spring and/or seal having to be used.
A further advantage resides in the fact that the seal is optimally positioned. It lies directly in front of gaps which are present and is pressed in the direction of the gaps by the spring action of the spring. A great pressure acting on the spring because of a medium whose pressure is to be measured reinforces the spring action in this direction. It is therefore not possible for the seal to become disengaged, because of a pressure-induced deformation of the seal, even when high pressures are exerting an effect.
The invention and further advantages will now be explained in greater detail with reference to the figures of the drawing in which two exemplary embodiments are illustrated.